This invention relates generally to fuel compositions for combustion turbines, and more particularly, to fuel compositions and methods for extending the time between turbine washes when burning ash bearing fuels.
When combusted in a turbine, various inorganic constituents (ash) in a fuel can affect turbine operation particularly over extended periods of time. Certain constituents in a fuel can cause corrosion of the various parts of the turbine. Other constituents in a fuel can form non-corrosive deposits on the various parts of the turbine. Deposits are often periodically removed with a standard turbine wash cycle.
For example, sodium, potassium, and vanadium are of concern in hot corrosion. Turbine manufacturers typically recommend less than 1 ppm (parts per million) by weight of sodium and potassium combined, less than 0.5 ppm by weight of vanadium, and less than 5 ppm other trace metals by weight. In particular, vanadium in a concentration greater than 0.5 ppm by weight forms low melting point vanadium compounds which have been implicated in hot corrosion. Magnesium compounds have been added to fuels having a vanadium content greater than 0.5 ppm by weight to reduce vanadium corrosion in gas turbines. The magnesium compounds react with vanadium to form solid magnesium vanadates, capturing the vanadium in an inert chemical state.
In contrast, other constituents in the fuel form deposits on and foul various components such as turbine nozzles and buckets in the turbine hot gas path. Over extended periods of operation, the deposits can build up and partially block the flow of hot gas through the turbine. A typical turbine can tolerate a ten-percent blockage of the flow of hot gas before the turbine must be taken off-line for cleaning. Failure to remove the deposits eventually leads to compressor surge, i.e., extreme vibrations, which causes a shutdown of the turbine.
Turbine manufacturers typically recommend that the maximum ash content by the fuel be no more than 50 ppm by weight. Calcium, for example, is known to form relatively hard to remove deposits, with the hardness of the .deposits increasing with increased firing temperatures; Of the total ash content, turbine manufacturers typically recommend that the content of calcium be no more than 2 ppm by weight in true distillate fuel oils, and no more than 10 ppm by weight in ash bearing fuels, e.g., crude and blended residual fuels and heavier residual fuels.
In order to meet these recommended levels, undesired inorganic constituents in ash burning fuels for use in turbines are often removed from the fuel by water washing and filtration. However, there are ash bearing fuels which cannot be made to comply with the manufacturer""s fuel specification, even with multiple stages of water washing.
There is a need for fuel additives which will extend the wash interval, and improve the wash effectiveness, when burning ash bearing fuels that contain ash constituents which are generally not corrosive, but which form hard-to-remove deposits.
The present invention, in one aspect, provides a composition which includes an ash bearing fuel having less than 0.5 ppm vanadium by weight, at least one of greater than about 25 ppm ash by weight and greater than 2 ppm calcium by weight, and an additive comprising a magnesium compound.
In a second aspect, a liquid petroleum ash bearing fuel composition for extending the time between turbine washes when burning ash bearing fuel includes an ash bearing fuel having less than 0.5 ppm vanadium by weight, less than 1 ppm sodium and potassium combined by weight, at least one of greater than about 25 ppm ash by weight and greater than 2 ppm calcium by weight, an additive comprising a magnesium compound, and wherein a weight ratio of magnesium to ash is greater than about 0.5 to 1.
In a third aspect, a method for operating a turbine includes providing an ash bearing fuel comprising less than 0.5 ppm vanadium by weight, and at least one of greater than about 25 ppm ash by weight and greater than 2 ppm calcium by weight, providing an additive comprising a magnesium compound, mixing the ash bearing fuel and the additive and burning the mixture of the ash bearing fuel and the additive in the turbine.
In a fourth aspect, a method for extending the time between turbine washes when burning ash bearing fuel in a turbine includes providing an ash bearing fuel having less than 0.5 ppm vanadium by weight, less than 1 ppm sodium and potassium combined by weight, and at least one of greater than about 25 ppm ash by weight and greater than 2 ppm calcium by weight, providing an additive comprising a magnesium compound, mixing the ash bearing fuel and the additive so that a weight ratio of the magnesium to ash is greater than about 0.5 to 1, and burning the mixture of the ash bearing fuel and the additive in the turbine.
Advantageously, the addition of an additive comprising magnesium to ash bearing fuel also improves the effectiveness of a standard turbine wash cycle when burning the ash bearing fuel in a turbine.